People all across the country are being told on a daily basis that their thyroid gland just died or stopped working. Some patients just accept this without question. Others ask how this could happen. The usual response by their doctor is that it is just something that happens. Almost as if the thyroid gland was a light switch that could just be turned off. Thyroid glands do not just stop working and die for no apparent reason.
Well, because the f/stop is a ratio of the focal length to diameter, our 50mm lens at f/2.8 would have a diameter of 50/2.8 = 17.86mm. Remember, we have to divide that by 2 to get the radius of 8.93mm, so the area of the circle thus formed would be Ï X 8.932, or 250.5 square mm.
Point objects at distances and would be in focus at image distances and, respectively; at image distance, they are imaged as blur spots. The depth of field is controlled by the aperture stop diameter; when the blur spot diameter is equal to the acceptable circle of confusion, the near and far limits of DOF are at and.